How Are Plants Propagated?

Reproduction is a basic property of all life. Plants reproduce several ways,
and humans have taken advantage of natural reproductive systems to increase
the number of plants for human use. There are two major types of reproductive
methods: a) Sexual and b) asexual reproduction.

Sexual Reproduction

Sexual reproduction involves the fusion of two cells (gametes) to
produce a new cell (zygote) which will develop into the new individual.
In animals such as humans and also in plants, sexual reproduction involves
relatively large, immobile gametes (eggs), which are produced by females
and small, motile gametes (sperm), which are produced by males.

Some groups of plants produce seeds during the sexual reproductive process.
The most economically important plants, those plants with which we are most
familiar, the flowering plants, produce seeds.

Seeds are produced within flowers of flowering plants: some plants may produce
hundreds, even thousands of seeds, during a lifetime. Each seed contains
one immature plant (embryo) within it. The embryo developed from a
fertilized egg, and therefore resulted from sexual reproduction.

Once mature, a seed possibly may remain dormant for up to many years; in
the dormant condition the embryo remains relatively unchanged. When a seed
is transported to a suitable environment, chemical changes occur within the
seed and the embryo initiates growththis is seed germination.

The new plant that develops from the embryo when a seed germinates resulted
from the fusion of an egg and sperm during the process of fertilization.
The egg was produced by one parent and the sperm by another. Since the new
plant that grows from a seed contains contributions from two different parents
the new plant is not identical to either parent. This is a very important
point about the offspring produced in sexual reproduction: The new individual
is not identical to either parent or to any of its siblings! (Identical twins
are the exception to this rule but are explained by the splitting of one
young embryo into two.)

Seeds, the major method of reproduction for most of our economically significant
plants (e.g., corn, wheat, rice, and soybeans), are products of sexual
reproduction. New individual plants produced when seeds germinate may be
very similar to their parents and siblings, but the new individuals are not
identical to siblings or either parent.

Asexual Reproduction

In addition to producing new individuals by seeds, many plants produce new
individuals without the sexual process: This is referred to as asexual
reproduction. Most of the ways that plants reproduce asexually fit into what
many botanists refer to as either vegetative propagation or cloning. In this set of on-line readings we consider cloning, vegetative propagation, and
asexual reproduction as synonyms.

Modified stems are several examples of how plants reproduce asexually. Bulbs
and corms are a common way that plants produce new individuals
naturally, and what humans take advantage of for increasing the number of
individual plants commercially. In the late summer and early autumn garden
stores, department stores, grocery stores, and even some gasoline stations
sell bulbs and corms for fall planting. If you plant a bulb of a daffodil
bulb and leave it undisturbed, you should notice that after several years
a bunch of identical daffodils will flower where you originally planted one
bulb. If you carefully dig up the original bulb you would find that there
are many small bulbs around it. The original plant had produced new bulbs,
hence new plants, asexually. If you were in the business of selling daffodil
bulbs you would have increased your inventory just by letting the original
plant reproduce asexually.

The important point here is that the new individuals are identical to the
original plants. This is a major difference between vegetative propagation
and propagation by seeds: New individuals produced by vegetative propagation
are identical to the parent and to the siblings.

This is an important point when a plant has desirable characteristics such
as a delicious fruit, attractive flower, resistance to disease, or unusually
rapid growth. Cloned plants are very common commercially since offspring
produced from a plant with desirable traits will be identical to it.

Major Methods of Vegetative Propagation

Stems, roots, and leaves are used for cloning plants. The following are some
of the major ways that plants are cloned in addition to bulbs which were
discussed in the preceding section.

Many plants can be cloned with cuttings. A portion of stem with leaves
is cut from the parent and placed in a suitable rooting medium for that
particular species of plant. Some suitable media include moist sand, a mixture
of peat moss and soil, or water. After roots have developed from the cut
end of the stem the cutting is transplanted to soil. House plants such as
coleus, philodendron, ivy, and geraniums commonly are propagated by cuttings.

Grafting is similar to cuttings in that a portion of stem or in some
cases a bud is removed from the parent. The part removed from the parent
is called a scion. The scion is then attached (i.e., grafted) to a
stock, which is either a stem or root of another plant. In making
a graft it is important to match the vascular cambium of the scion to that
of the stock. A graft will fail if the cambia are not in contact. Grafting
is used when it is difficult to root cuttings of the plant being propagated,
or if the stock has desirable traits not found in the plant that is the source
of the scion. For example, roses are grafted onto root stock of wild roses.
The above ground part of the rose has the qualities we like in a rose (pretty
flowers and pleasing aroma), and the roots resist harsh winters which the
original parent of the scion could not do. Grafting is used to combine desirable
traits of the scion and stock in grapes also: The scion produces the kind
of grape that is desired (e.g., seedless and sweet), and the root stock is
resistant to insects that damage roots of many plants. Grafting is also an
important means for reproducing plants that do not produce seeds. The best
examples here are seedless navel oranges, seedless grapefruit, and seedless
grapes.

Rhizomes are used commercially to propagate common plants such as
irises and ferns. As the underground stem grows, it branches, thus forming
new apical meristems. Rhizomes can be broken or cut into several pieces to
increase the number of individuals. Some ferns may attain a very large size
through the growth of rhizomes. If one were to observe what is originally
believed to be a population of ferns on a hillside, the many groups of leaves
may be in reality all part of the same plant with an underground stem, the
rhizome. Some ferns called "bracken" in Europe may cover an area over 390
meters in diameter, an illustration that one rhizome may be extensive.

Runners and stolons also increase the number of plants asexually.
The original plant sends out the modified stems, runners/stolons, which produce
new upright branches and roots at the nodes. If the runner/stolon is broken
or cut, new individuals are the result. Runners and stolons are used commercially
for mints, strawberries, and many grasses.

Tubers, the swollen tips of underground stems, are the major way that
potatoes are propagated. Each eye of the potato is a bud at a node. If a
small portion of a potato with an eye is planted, a new plant will be produced.

We saw with cuttings that roots can develop from the cut end of a stem. Many
plants have roots that will produce a shoot if the root is separated from
the rest of the parent plants. Common examples include black locust trees
which have roots that readily send up shoots, and sweet potato which are
large roots (taproots) that will produce shoots. Anyone who tries to remove
weeds from a lawn also is aware of another example of roots that produce
shoots: Any missed part of the taproot of a dandelion will develop a new
shoot.

Leaves of some plants have the ability to produce both shoot apical buds
and roots. In nature these plants increase the number of individuals if a
leaf falls onto the ground. New shoots and roots develop and thus a new,
cloned individual results. Commercially, plants with leaves that readily
form shoots and roots are cut into several pieces so that several plants
can be produced from a single leaf.

Technical Advances in Cloning

Methods for cloning plants discussed in the preceding section take advantage
of the natural abilities of plants to asexually produce new individualsthe
horticulturist merely is helping each plant with what it already does. There
are now new ways to clone plants that are in addition to what plants normally
would do. In the past few years plant scientists have begun to use methods
of biotechnology for commercially producing cloned plants.

Basic studies of plant development have shown that one plant cell can give
rise to an entire, new plant if the cell first is grown into a mass of cells
(callus) under a controlled environment and the callus then treated
with special chemicals (plant hormones) that induce development of shoots
and roots. Using these tissue culture methods, one parent plant could be
divided into thousands of groups of cells, each of which could develop into
a callus. Each callus also could be divided into thousands of groups of cells,
each of which could develop into a callus. By repeatedly dividing a callus
into individual cells and growing each cell into a new callus, in a relatively
short time (e. g., several months) from 100,000 to a million potential individual
plants would be produced from the original parent. When the time is right,
the proper combination of hormones is added to the thousands of calli and
roots and shoots develop on each. The young plantlets are transferred to
soil and grown to the stage of development for marketing. Since the process
of propagating the new plants did not involve sex, each new plant produced
is identical to its siblings and to the parent.

Tissue culture methods for plant propagation are carried out in a laboratory.
Cells are separated and calli grown under sterile conditions, so the plants
that are produced are absolutely disease free. Tissue culture methods continually
are being improved and we are seeing more plants each year produced by tissue
culture. Douglas fir for tree farms, ferns, African violets, and day lilies
are several plants currently propagated commercially through tissue culture.

Summary

The main points to remember about plant propagation are that 1) sexual
reproduction results in new individuals that are not identical to siblings
or the parents, and 2) asexual reproduction (cloning or vegetative propagation)
produces offspring that are identical to each other and the parent. Seeds
are the products of sexual reproduction. Major examples of plant parts used
in cloning include bulbs and corms, rhizomes, runners and stolons, tubers,
and taproots. Tissue culture involves recent advances in plant biotechnology
for producing many disease-free individuals from one parent.